1. Field of the Invention
This invention relates to an epitaxial substrate for a compound semiconductor light-emitting device composed of stacked layers of nitride-system III-V Group compound semiconductor thin film, a method for producing the same, and a light-emitting device.
2. Background Art
III-V Group compound semiconductors represented by the general formula InxGayAlzN (where x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1) are known as materials for purple, blue and green light-emitting diodes and blue and green laser diodes. In the following, the symbols x, y and z appearing in this general formula are sometimes used to denote InN mixed crystal ratio, GaN mixed crystal ratio and AlN mixed crystal ratio, respectively. III-V Group compound semiconductors, particularly those containing InN at a mixed crystal ratio of 10% or greater, are especially important for display applications because their emission wavelength in the visible region can be regulated by adjusting the InN mixed crystal ratio.
It is known that the physical properties of compound semiconductors are strongly affected by their mixed crystal ratios. For example, a GaAlN-sytstem mixed crystal containing no In has excellent thermal stability and can therefore be grown at a temperature of 1,000° C. or higher to obtain good crystallinity. In contrast, the thermal stability of an InGaAlN-system mixed crystal (containing In), though differing with the InN mixed crystal ratio, is not so good, so this compound semiconductor is generally grown at a relatively low temperature of around 800° C. As a result, the InGaAlN-system mixed crystal growth layer constituting an important growth layer of the light-emitting layer structure of a visible-light-region light-emitting device has to be grown at a low temperature and is therefore frequently inadequate in thermal stability.
On the other hand, once the light-emitting layer structure has been grown, it is overlaid with a p-type layer that has to be grown at a high temperature. In order to protect the thermally unstable light-emitting layer structure from this high growth temperature, the conventional practice has therefore been to first grow a protective layer having high heat resistance on the light-emitting layer structure and then grow the p-type layer on the protective layer at a high growth temperature.
This protective layer is an important layer that not only works to protect the light-emitting layer structure but also has a strong effect on the emission characteristics of the light-emitting device. Specifically, it is deeply involved in the process whereby light emission is caused by recombination of holes effectively injected into the light-emitting layer structure from the p-type layer formed on the protective layer with electrons injected into the light-emitting layer structure from its lower side. In order to enhance the efficiency of hole injection into the light-emitting layer structure from the p-type layer side, it is preferable for the protective layer to have p-type conductivity or n-type conductivity of low carrier density.
However, when a protective layer containing Al is grown at the same temperature as that at which the light-emitting layer structure was grown, the crystallinity of the protective layer is not good. Owing to the presence of many lattice defects, therefore, it exhibits high n-type conductivity of ordinary carrier density. The injection efficiency of holes into the light-emitting layer structure is therefore lowered and it becomes difficult to achieve high light-emission efficiency.
The protective layer and p-type layer growth conditions therefore have to be optimized from the two aspects of improving protective performance and maintaining hole injection efficiency. The protective layer is required to protect the light-emitting layer structure from heat and, while maintaining high crystal quality, to control conductivity for ensuring efficient hole injection from the p-type layer into the light-emitting layer structure.
The prior art provides two methods for enabling this optimization: (a) the method of first growing a protective layer of AlGaN, then heating the protective layer to the growth temperature of a p-type GaN layer (contact layer), and finally growing the p-type GaN layer, and (b) the method of forming a protective layer as an Al-free GaN layer growable at a low temperature, thereby enhancing the crystallinity of the protective layer and making the background-type carrier density somewhat low.
However, when method (a) is adopted, the protective layer must be grown to at least a given thickness to prevent degradation of the p-type GaN layer (contact layer) in the course of growth. Since the resulting increase in layer thickness separates the pn junction interface from the light-emitting layer structure, the efficiency of hole injection into the light-emitting layer structure decreases. Moreover, owing to the fact that the performance of the AlGaN layer as a protective layer declines with decreasing Al content, the GaN protective layer of the prior art method (b) also has to be increased in thickness, so that no improvement in light-emission efficiency can be expected.